7/17/2018 1:00 AM PDT

Luann de Lesseps going back into rehab might have shocked fans, but the people closest to her knew she was only pretending to be sober for weeks now ... TMZ has learned.

Sources close to the 'Real Housewives' star tell us she fell off the wagon at least 2 weeks before Bethenny Frankel announced her 'RHONY' pal was reentering a rehab facility -- the second time in less than year.

We're told friends point to a July 2 pic Luann posted with her ex-husband, declaring she'd been sober for 6 months -- no small feat for someone in recovery.

However, the very next day things fell apart rather publicly at a charity event in the Hamptons. Our sources say Luann made a late and noisy entrance -- disrupting a piano performance -- then was seen hitting up the bar, and hitting on several guys in a very touchy-feely way.

We're told friends think there are a number of reasons she slipped. One being her Christmas Eve arrest in Florida, but more recently she was sued by her 2 adult children for allegedly screwing them out of a trust.

Sources say the family divide weighed heavily on Luann. We've reached out to her camp, but no word back, yet.

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